1. Field of the Invention
The present invention relates to a motor control device.
2. Description of the Related Art
There is known a direct current (DC) motor that rotates at a rotation speed proportional to an applied voltage. As a kind of such a DC motor, there are known a brush motor and a brushless motor. The brushless motor is free from brush wear and has high durability, and is thus used frequently. Under feedback control using a position/speed sensor of an encoder, and other components, the DC motors are controlled to stop (speed zero) at arbitrary timing or controlled to an arbitrary target speed.
Japanese Laid-open Patent Publication No. 2009-148082 discloses an image forming apparatus that aims at improving image quality by adjusting (tuning) a gain to an appropriate value. In the image forming apparatus, an error detection unit determines whether a frequency error exists. When a frequency error exists, the error detection unit further determines whether the frequency error is within a target error range. When the frequency error is within the target error range, a gain (P gain) of a proportional term of proportional integral control is reduced. When the frequency error is not within the target error range, the P gain is increased. The frequency proportion integral and differential (PID) arithmetic processing is performed based on the set gain.
However, the response characteristics (change of a revolving speed in accordance with an applied voltage) of the DC motors such as a brushless motor change significantly depending on a rotation speed. When the rotation speed is changed, the total feedback characteristics are also changed. Thus, when the rotation speed is controlled using a fixed feedback gain, for example, it is difficult to stably control a rotation speed in a wide range.
Moreover, in the case of the image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2009-148082, the proportional gain is variable in accordance with a target rotation speed. Thus, when the motor is not rotated at a speed around a target rotation speed, the rotation can be instable. The gain corresponding to the rotation speed is determined considering only rotation fluctuation. Therefore, it is difficult to optimize response characteristics (quickness in following when a target speed is changed) and stability.
In view of the above, there is a need to provide a motor control device capable of controlling a rotation speed of a motor stably and optimally.